Fix possible deadlock in GILAwareManualExecutor

Summary:
See code comment

(Note: this ignores all push blocking failures!)

Reviewed By: andriigrynenko

Differential Revision: D8130722

fbshipit-source-id: c2a3e4b0de1fe7d07e28c4a7558e4367a1591ac1

folly/python/GILAwareManualExecutor.cpp

@@ -40,6 +40,9 @@ void GILAwareManualExecutor::waitBeforeDrive() {
   // Release GIL before waiting on lock
   auto* pyThreadState = PyEval_SaveThread();
   SCOPE_EXIT {
+    // Release lock before re-acquiring GIL,
+    // to avoid deadlock if another GIL-owning thread is calling add
+    lock.unlock();
     PyEval_RestoreThread(pyThreadState);
   };
   cv_.wait(lock, [&] { return !funcs_.empty(); });